1. Field of Invention
The present invention relates to the conversion of chemical energy to electrical energy, and more particularly to a mixture of polymeric binders for construction of an electrode intended for incorporation into an electrochemical cell. The preferred binder formulation is a mixture of a halogenated polymeric binder and a polymide binder, most preferably polyvinylidene fluoride (PVDF) and a polyimide. Electrodes built containing the mixed polymeric binders are useful in both primary and secondary cells discharged at elevated and low temperatures.
2. Prior Art
Halogenated polymeric materials have been used extensively as binders for electrodes in all types of nonaqueous electrochemical cells, and particularly lithium cells. For example, polyvinylidene fluoride is a material that functions well as an electrode active binder at or near ambient temperature. However, PVDF is soluble in organic electrolytes at elevated temperatures. Thus, cells manufactured with halogenated polymeric materials such as PVDF as the sole binder material cannot be used in high temperature applications or survive high temperature exposure, such as occurs in an autoclave, without severe degradation.
It is also known in the prior art to employ non-halogenated polymeric materials as binders in nonaqueous, alkali metal electrochemical cells. Exemplary is U.S. Pat. No. 5,468,571 to Asami et al. which discloses that polyimide (PI) is useful as an anode binder in lithium secondary cells. Electrodes containing such non-halogenated polymers as the sole binder material are somewhat brittle and have a tendency to crack.
U.S. Pat. No. 6,001,507 to Ono et al. describes electrodes for secondary cells prepared from a mixture of a soluble polyimide and PVDF combined with an active material such as LiCoO2. The soluble polyimide is a material which is converted to the imide before it is mixed with the depolarizing active mixture. This is done to prevent water from entering the electrochemical system. However, fabricating an electrode with binders which are soluble in nonaqueous solvents hinders the active mixture/current collector contact interface, especially after repeated cycling.
Thus, there is a need for a binder formulation that is insoluble in both primary and secondary organic solvent electrolyte systems, particularly those used to activate alkali metal primary cells or alkali metal ion secondary cells, and is able to withstand high temperature exposure without compromising discharge efficiency.
The present invention is, therefore, directed to a mixture of two polymeric binders that are insoluble in nonaqueous organic electrolytes activating alkali metal and alkali metal ion electrochemical cells and that provide flexible, non-brittle electrodes dischargeable at elevated temperatures. The first binder is preferably a halogenated polymeric binder, and more preferably a fluorinated polymeric material, such as PVDF. The second binder is polyimide, and preferably one derived from a polyamic acid prior to activating the electrochemical couple. A preferred binder mixture is PVDF and PI. An electrode comprising such a binder mixture can serve as the cathode in a primary alkali metal electrochemical cell or as the negative electrode in a secondary cell, such as a lithium ion cell.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description and the appended drawings.